In recent years, the so-called shoe press for dehydrating a wet web by pressing one surface of the wet web placed on a felt member traveling at a high speed with a press roll while pressurizing the other surface with a pressure shoe through an endless belt has been widely used in the press portion of a papermaking step, in order to improve the dehydration efficiency for the wet web. In the shoe press, a belt endlessly formed by integrating a reinforcing substrate and an elastic material such as thermosetting polyurethane with each other is employed. Also in a calender step of smoothing and glossing the surface of paper, employment of an elastic belt similar to the aforementioned one has been recently studied. In addition, employment of a similar elastic belt is studied also as to a sheet transfer belt for preventing a web break and for stably transporting a wet web, particularly when performing papermaking at a high speed. Japanese Utility Model Laying-Open No. 59-54598 or the like discloses a typical structure of such a papermaking belt prepared by forming elastic materials on both surfaces of a fabric base and providing grooves on the surface.
Generally in shoe pressing, for example, severe bending and pressing are repeated on the belt between the press roll and the pressure shoe, and hence the elastic material forming the belt is unavoidably cracked when the belt is used over a long period of time. In general, such cracking is generated from the outer peripheral surface of the belt. A crack once caused progresses into a large crack as the belt is used. When the crack progresses, lubricating oil stored between the inner peripheral surface of the belt and the pressure shoe externally leaks to exert a bad influence on paper or cause delamination of the belt.
On the other hand, the fabric base may be broken by repetition of bending and pressurization in advance of cracking, to cause delamination. Further, delamination may be caused by weakness of adhesion between the fabric base and the elastic material. Thus, the progress of a crack, breakage of the fabric base, weakness of the adhesion between the fabric base and the elastic material etc. reduce the life of the belt.
In order to improve the life of the belt, a solution for the problem of delamination of the belt has been studied. In order to solve the problem of delamination of the belt, Japanese Patent No. 2889341 proposes a dehydration press belt obtained by forming an intermediate elastic layer at least on one surface of a fabric base layer, further forming a surface elastic layer and a back elastic layer on the outer side of the intermediate elastic layer and the other surface of the fabric base layer respectively, and bonding and integrating these elastic layers to and with each other.
According to the aforementioned technique, air remaining in the fabric base layer can be expelled when the fabric base layer is coated with the intermediate elastic layer in the process of manufacturing the belt. Since air can be expelled from the fabric base layer, a belt containing no pinholes between the fabric base layer and the elastic layers can be manufactured. Since the belt can be manufactured to contain no pinholes between the fabric base layer and the elastic layers, an effect, to some extent, can be attained concerning the problem of delamination.
On the other hand, Japanese Patent No. 3045975 discloses a shoe pressing belt prepared by varying the hardness of the resin forming the belt to be high in a central region along the width direction and low in both edge regions for suppressing cracking at the shoe edge portions. However, since the belt disclosed therein has only two elastic layers, i.e., a surface layer and a back layer holding a fabric base therebetween, it is not easy to manufacture a belt not which does contain pinholes, and hence the problem of delamination may still remain. When the central region along the width direction is cracked further, this crack disadvantageously progresses toward the fabric base.
Japanese Patent No. 2,542,250 discloses still another typical structure obtained by embedding reinforcing yarns in an elastic material. Further, U.S. Pat. No. 5,943,951 discloses a belt formed with flexible parts on ends of a press region for preventing cracking on shoe ends in a structure obtained by embedding reinforcing yarns in an elastic material. However, in this belt, when the central region in the width direction is cracked, this crack may also progress into the belt. In particular, when reinforcing yarns are embedded in an elastic material a crack may progress into the inner peripheral surface of the belt when the surface of the belt is cracked.